Increasing health consciousness, due to recent issues of air pollution and infectious disease epidemic, has spurred demands for air cleaners, automotive cabin filters, and the like. These devices employ a common technique, which uses an air filter medium composed of a nonwoven fabric or the like to remove air dust. Such an air filter medium is required to have a high collection efficiency.
A high collection efficiency of a filter medium composed of a nonwoven fabric is known to be achieved via a technique of reducing the diameter of fibers constituting the nonwoven fabric (see Patent Literature 1). However, in this method, as the filtration performance is improved, the pressure drop of the filter medium increases. When the pressure drop increases, energy requirements for air cleaning and filtration increases.
The above problem is widely known to be solved by subjecting the fibers to charge treatment. The charge treatment, also called electret treatment, is an extremely effective means for increasing collection efficiency for microparticles in the air (see Patent Literature 2, 3 and 4). This technique can greatly improve the collection efficiency. However, there has been an increasing need for further reduction of pressure drop of filters and therefore there has been a demand for a technique for achieving a much lower pressure drop.
As such a technique for achieving a much lower pressure drop, proposed are methods using, as a filter medium, a nonwoven fabric comprising a mixture of two or more types of charged fibers having different fiber diameters (see Patent Literature 5, 6 and 7). In addition to the methods using charged fibers, proposed are methods using a combination of fibers having different fiber diameters to improve the filtration performance (see Patent Literature 8 and 9). Among them, Patent Literature 7 describes that a meltblown nonwoven fabric comprising a mixture of microfibers having a fiber diameter of 10 μm or less and larger size fibers having a fiber diameter of more than 10 μm has appropriate sheet formability and breathability for applying to masks or filters.
There are various methods for producing a mixed-fiber meltblown nonwoven fabric comprising a plurality of materials. An example of such a production method of a nonwoven fabric for filters is a method comprising melting one type of fibers to increase the durability of a resulting nonwoven fabric (see Patent Literature 10). Another example of a similar production method, whose main purpose is not the production of a nonwoven fabric for filters, is a method comprising mixing different fibers, at least one type of which is made of an elastomer, to improve the elasticity and feel of a resulting nonwoven fabric (see Patent Literature 11, 12 and 13).